1. Field of the Invention
The present invention relates to electrohydraulic systems for powering components on a vehicle, and more particularly to a distributed hydraulic system having multiple actuators operated by a plurality of electronic controllers that exchange control messages over a communication network on the vehicle.
2. Description of the Related Art
With reference to FIG. 1, a backhoe 10 is a well known type of earth moving vehicle that has a bucket 12 rotatably attached to the end of an arm 14 that in turn is pivotally coupled by a boom 16 to a tractor 18, thereby forming a boom assembly 15. A hydraulic boom cylinder 21 raises and lowers the boom 16 with respect to the tractor 18 and a hydraulic arm cylinder 22 pivots the arm 14 about the end of the boom. The bucket 12 is rotated at the remote end of the arm 14 by a hydraulic bucket cylinder 23.
Traditionally, the boom assembly 15 is controlled by valves located within the cab of the tractor 18 and mechanically connected to levers which the operator manipulates to independently move the boom, arm and bucket. A separate valve is provided for each of the cylinders 21–23 on the boom assembly 15. Operating one of the valves controls the flow of pressurized hydraulic fluid from a pump on the tractor to the associated cylinder and controls the return of fluid from that cylinder back to the tank on the tractor. A separate pair of hydraulic conduits runs from each cylinder along the boom assembly to the respective valve in the operator cab. Each of these conduits is subject to fatigue as they flex with motion of the boom assembly.
There has been a recent trend away from mechanically operated valves to electrohydraulic valves that are operated by electrical signals. Electrical valve operation enables computerized control of the functions on the machine. In addition, hydraulic control now can be distributed throughout the machine by locating the valves for a given hydraulic function in close proximity to the hydraulic actuator, such as a cylinder or motor for example, being operated by those valves. Such distributed control reduces the amount of plumbing on the machine. A single hydraulic fluid supply conduit and a single fluid return conduit connect all the valve assemblies to the pump and tank on the tractor 18.
The operator in the cab of the tractor 18 with a distributed hydraulic system manipulates joysticks or other input devices to generate electrical control signals for operating the valve assemblies located adjacent each of the boom assembly cylinders 21, 22 and 23. U.S. Pat. No. 6,718,759 describes a velocity based system for controlling a hydraulic system with multiple function in which a velocity command is produced for machine functions in response to the corresponding joystick signals. The velocity command and other signals for a given machine function are transmitted over a shared communication network to a separate function controller which is associated with the valve assembly that controls the hydraulic cylinder for that machine function. Each function controller is located in close proximity to the associated valve assembly. The function controllers also send data and other messages over the communication network to the system controller.
A common communication network 56 used in vehicle control systems is the Controller Area Network (CAN) defined by the ISO 11898 standard, promulgated by the International Organization for Standardization in Geneva, Switzerland. In addition to servicing the hydraulic system, the communication network also carries commands and data regarding operation of the engine, transmission and other components on the vehicle. The advantage of using an standardized communication network, as compared to a network that uses a proprietary communication protocol, is that vehicle devices from many manufacturers are able to communicate over that network. However, a drawback of a standardized communication network is that protocol parameters are fixed and can not be varied to meet the requirements of a given device manufacturer. With a distributed hydraulic control system, for example, the data transmission rate can not be changed to enable a greater amount of messages to be communicated between the various controllers in a given time period. Therefore the communication network 56 has a finite bandwidth that limits the number of messages that it is able to carry. As a consequence, if numerous devices are competing for access to the network in order to send a message, a given device may have to wait a relatively long time before sending its message and that message may not arrive at the recipient device in a timely manner. Thus feedback signals and other operations may be delayed which erode the robustness of machine performance.